Maxtrac repeater power foldback issue

[mancow]

I am running a pair of maxtracs here with the the flat audio lines connected through a shared repeater tone panel controller set to CSQ mode. It repeats P25 CAI and D-STAR just fine. The transmit radio is connected to a Milcom 160 watt amp that has been modded with a 30 watt intermediate amplifier board. The Milcom was originally a 40 watt drive amp so the small board takes care of that step up so that the transmit maxtrac radio doesn't have to be driven so hard on transmit.

I don't have a watt meter handy right now but the relative power output of the Maxtrac is set at 35 (this is a vhf radio for info). The transmit radio has a fan mounted to the back that kicks on in conjunction with the large Milcom amp's fan. The transmit radio seems to be running fairly cool. I mean it's barely even warm to the touch on the back after several minutes of use. The problem is that the amp meter on the power supply will drop after several minutes of use and the power output from the Milcom will drop off to nothing. It's as if something is folding back after a while but nothing seems to be that warm. The entire system is only drawing 10 amps right now so I know nothing here is really pushing that much power at all.

I know this is an impossible problem to chase down without a watt meter but I guess my main question is this; Does the Maxtrac have some sort of timer or other circuit controlled by something other than temperature that causes it to fold back power after a while?

[Will]

You need to check the control voltage going to the Maxtrac PA. Maxtracs do not have temperature fold back.

[kcbooboo]

"I know this is an impossible problem to chase down without a watt meter but I guess my main question is this; Does the Maxtrac have some sort of timer or other circuit controlled by something other than temperature that causes it to fold back power after a while?"

Yes, it DOES fold the power back to zero after about 45 minutes of continuous (i.e. repeater) transmission. There's no temp sensor on the PA feeding the power control circuit. The GM300 does have a temp sensor as part of the power controller, so it may lower the power if things get too hot.

You have a couple of options: use a GM300 instead of a MaxTrac, or add manual power control to the MaxTrac. There's an article about it on another web site, but since no one reads anything over there, I won't bother mentioning the site.

Bob M.

[mancow]

LOL...well you've addressed two of my questions today and I will definitely go check out that "other site".

thanks

[Max-trac]

Different bands and models seem to drop off differently.
The lowband drops off in timed steps of about 10 watt increments.
Just put a manual power pot in it!!!
You will have to play with the resistor divider values as each radio is different.
Then you can get a nice wide swing on the pot, IE you don't want full to off in only a couple of degrees as it will drift too much.

[mancow]

That's pretty much what it's doing, especially at first when I was running the radio at a higher power level. It would drop at about 10 watt increments to zero. The seemingly strange thing is that it did that when the radio was really fairly hot to the touch. Now that I have the power folded way back the radio barely gets warm. However, the power now drops off sharply after maybe 5 -10 minutes of talk time with a few small breaks in between.

I guess I'll go looking for that line to install a manual adjustment component.

[PETNRDX]

I have also found that some Maxtracs have done the "foldback" based on TX time.

Some have been pretty quick, like your 5 or 10 minutes.

Put in the manual adjust pot and it fixes the problem.